David Heinold AECOM Chelmsford, MA. Ambient Air Modeling Workshop NCASI Southern Regional Meeting Savannah, Georgia

Transcription

1 David Heinold AECOM Chelmsford, MA Ambient Air Modeling Workshop NCASI Southern Regional Meeting Savannah, Georgia June 9, 2014

2 Acknowledgement The contents of this presentation is based on work conducted by AECOM on behalf of the Electric Power Research Institute (EPRI) EPRI Program Director: Dr. Eladio Knipping AECOM Project Manager: Robert Paine EMVAP is a public domain tool and is provided at EPRI s website at June 9,

3 EPA Policy on Averaging Time for SO 2 Emission Limits Historical EPA guidance SIP emissions limits should not exceed the averaging time of the applicable NAAQS and be set to the critical emission value Constant hourly emission rate that modeling would result in the 99 th percentile of daily maximum hourly SO 2 concentrations at the 1-hour NAAQS level New EPA Guidance (4/23/2014) States may try to develop control strategies that account for variability in 1-hour emissions rates through emission limits with averaging times as long as 30 days. Long-term average emissions rate < critical emission value June 9,

4 Proposed Statistical Method to Develop Long-Term Emission Limits for SO 2 Premise: Based on the known or projected variability of hourly emissions, a conservative probabilistic relation can be established between the long-term average emission rate and modeled 1-hour SO 2 NAAQS design value. EPRI s Emissions Variability Processor (EMVAP) Incorporates AERMOD modeling in the form of a post-processor Randomly samples emissions from a specified annual emissions distribution Provides probabilistic estimates of 1-hour SO 2 design concentrations EMVAP has been shown to be suitably conservative so as to provide a useful tool for regulatory applications. June 9,

5 How EMVAP Works Randomly samples emissions from a specified cumulative frequency distribution on an hour-by-hour basis to create many (e.g., 500 or more iterations) years of hourly emissions. Hourly emissions for each iteration are multiplied by hourly emission-normalized concentrations from AERMOD. Each iteration results in a design concentration. The 500 or more design concentrations are statistically analyzed to develop probabilistic estimates. June 9,

6 Step 1: Time Series of Hourly Emissions June 9,

7 Step 2: Cumulative Frequency Distribution June 9,

8 Step 3: Create Discrete Distribution June 9,

9 Step 4: Run AERMOD with Continuous 1 g/sec Emission Rate If emissions are strongly correlated with load Run AERMOD separately for up to 20 emission categories, each with specified exit velocity and temperature (same stack). If highest emissions are associated with a bypass stack Run AERMOD separately for up to 20 emission categories, each with specified exit velocity, temperature, diameter, height and location. Either 1 year of on-site or 5 years of off-site hourly meteorological data, per USEPA guidance can be applied. June 9,

10 Step 5: Apply EMVAP to Estimate Design Concentration Specify number of simulations 500 is recommended, but can accommodate thousands depending on number of receptors, years of meteorology and computer limitations Select one or more (up to 9) degrees of probability for the EMVAP-estimated design value 50 th percentile estimate provides the median estimate 95 th percentile provides an upper-limit estimate Consideration of the percentile level to select is provided later in this presentation June 9,

11 EMVAP Design Concentrations for 1000 Iterations 99%ile MDA1 (ug/m3) Simulation June 9,

12 Probability Distribution of EMVAP Design Concentrations 99%ile MDA1 50 th percentile Expected Value 95 th percentile Upper Limit 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Probability Distribution June 9,

13 EMVAP Design Concentrations Compared to SO 2 Measurements Data Bases that were Part of the AERMOD Evaluation Lovett Generating Station Clifty Creek Generating Station Kincaid Power Station June 9,

14 1600 SO 2 Design Value Comparison (µg/m 3 ) Monitored Modeled: Actual Emissions EMVAP 50th Percentile EMVAP 95th Percentile Clifty Creek Lovett Kincaid June 9,

15 Selection of EMVAP Percentile for Existing Sources Run EMVAP by selecting a range of percentiles Run AERMOD with actual hourly emissions Indentify the lowest of percentile design value that exceed the directly modeled design value Examples: For Clifty Creek and Lovett: 50 th percentile is suitable For Kincaid: greater than 50 th percentile but less than 95 th percentile is suitable Percentile selection and the corresponding degree of conservatism subject to negotiation. June 9,

16 Using EMVAP to Calculate Long-Term Average Emissions Rate 1. Determine the target design concentration (NAAQS level minus measured or modeled background). 2. Determine the 1-hour average critical emission value resulting in the target design concentration by applying AERMOD with constant emissions. 3. Develop or choose an emission distribution and corresponding long-term average emission rate applicable to future operations. June 9,

17 Using EMVAP to Calculate Long-Term Average Emissions Rate 4. Run EMVAP with this emissions distribution to determine the design concentration at a specified probability level (e.g., 50 th percentile) 5. Determine the compliant long term emission rate by multiplying the long-term average emission rate (Step 3 ) by the ratio of the target design concentration from (Step 1) to the EMVAP design concentration (Step 4). 6. If multiple sources or variable background are considered, rerun EMVAP with the compliant long average emission rates and iterate with adjustments to the average emissions for each source as needed. June 9,

18 Example Application Target SO 2 design value concentration: 157 µg/m 3 ( µg/m 3 background) AERMOD critical value emission rate: 327 g/sec Modeled emissions distribution EMVAP testing indicates that the 95 th percentile is appropriate June 9,

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20 Summary EPA s recently released guidance provides states the option of setting emission limits in their SO 2 SIPs based on averaging times as long as 30 days. EMVAP provides an objective means of establishing long-term emission rates that to a high and quantifiable degree of certainty will result in compliance with the 1-hour SO 2 NAAQS. The methodology would also applicable to new sources for which frequency distributions of hourly emissions are established. June 9,

21 Thank You For more information please contact: David Heinold, CCM AECOM June 9,